Such processes for producing so-called interference layer systems with a great variety of spectral properties are known in large numbers. As a rule, it is a matter of configuring the gradient of the curve between the properties of transmission and reflection as steeply as possible, in order to confer the rather special spectral properties upon the filter or mirror in question. Included among these are the higher-grade anti-reflection layers, as well as optical filters, vaporized metal oxide mirrors, etc.
As a rule, only one optical axis is present in the known process products. Said axis runs perpendicularly to the surfaces of the optical element, at least, however, perpendicularly to the central portion of the optical element, when it is a question of, for example, lenses or parabolic mirrors.
The reference wavelengths .lambda..sub.K and .lambda..sub.L should be understood to be those wavelengths, which are characteristic for the differentiable regions for the reflection and transmission behavior. Thus, for example, the reference wavelengths .lambda..sub.K of 482, 555, 653 and 752 nm, given in Table 1 infra, are for the visible range between 400 and 800 nm, while the reference wavelengths .lambda..sub.L for the longer wavelength region i.e. Table 2 infra from 2,000 to 20,000 nm is obtained only as a calculated quantity, which is checked by measurement of the finished product. The measurement and control procedures during the build-up of the succession of layers are carried out either by photometry with visible light or by nonoptical measurement procedures such as the piezoelectric resonator method.
The concepts of long wavelength and short wavelength are, for the present purposes, to be considered as relative to one another. In general, however, a short wavelength light radiation is understood to be one with a .lambda..sub.K between 400 and 800 nm (visible light) and a long wavelength radiation is understood to be an infrared radiation with a .lambda..sub.L between 2,000 and 20,000 nm.
The spectral region from 2,000 to 20,000 nm (or 2 .mu.m to 20 .mu.m) is of great significance, especially for infrared techniques. The optical elements used for this purpose must be transparent in the stated infrared spectral region. Materials, such as germanium, silicon and the non-oxide chalcogenides come into consideration for this. However, they have the disadvantage of a strong absorption in the visible region of the spectrum and therefore are opaque to the eye. Conversely, the conventional types of glass, which are transparent for the visible spectrum, absorb a very high proportion of the radiation in the infrared region, so that they do not come into consideration for this. Finally, the materials mentioned, which are transparent to infrared, have a very high refractive index, which leads to considerable reflection losses at the surfaces and reduces the transmission correspondingly.
However, there is the practical requirement to visually follow long wavelength radiation, which is invisible to the eye; this is possible only if the beam path of the long wavelength radiation is assigned a beam path for visible light. These two requirements, as noted above, cannot be combined, at least not without some difficulties.